US3916078A

US3916078A - Skid wire for pipe type electric cables

Info

Publication number: US3916078A
Application number: US505964A
Authority: US
Inventors: Paolo Gazzana Priaroggia
Original assignee: Pirelli SpA
Current assignee: Pirelli and C SpA
Priority date: 1973-10-12
Filing date: 1974-09-16
Publication date: 1975-10-28
Anticipated expiration: 1992-10-28
Also published as: JPS5066788A; GB1448607A; IT995803B; CA1006931A; IE39937B1; IE39937L; AR200943A1; BR7407945D0

Abstract

A pipe type electric cable in which the core comprising a central conductor surrounded by insulation which, in turn, is surrounded by a conductive screen, is received within a fluidtight metal tube and is separated from and conductively connected to the inner surface of the pipe by one or more skid wires wound around the screen, each of which skid wires comprises one or more metal cords extending longitudinally of the skid wire and surrounded by a semi-conductive synthetic resin which prevents contact of the cords with the inner surface of the pipe.

Description

United States Patent 1 Priaroggia SKID WIRE FOR PIPE TYPE ELECTRIC CABLES [75] Inventor: Paolo Gazzana Priaroggia Milan,

Italy [73] Assignee: Industrie Pirelli Societa per Azioni,

Milan, Italy [22] Filed: Sept. 16, 1974 [21] Appl. No.: 505,964

[30] Foreign Application Priority Data Oct 12, 1973 Italy 30035/73 [52] US. Cl. 174/10; 174/25 R [51] Int. C1. l-IOlB 7/22; H01B 9/06 [58] Field of Search 174/10, 24, 25 R, 25 G, 174/26 R, 26 G, 27, 108, 135, 136

[56] References Cited UNITED STATES PATENTS 2,516,747 7/1950 Bennett 174/25 R 2,615,074 10/1952 Bronovicki 174/10 2,665,328 l/l954 Atkinson et al 174/25 R 1 3,047,652 7/1962 Ege 174/25 R X Oct. 28, 1975 3,080,446 3/1963 Volk 174/25 R 3,579,863 5/1971 McGrath 174/10 3,594,492 7/1971 Bahder et a1. 174/108 X 3,621,110 11/1971 McGrath..- 174/10 3,673,307 6/1972 Eager, Jr. et al 174/25 R FOREIGN PATENTS OR APPLICATIONS 1,151,413 l/1958 France 174/25 R Primary Examiner-Arthur T. Grimley Attorney, Agent, or Firm-Brooks I-Iaidt I-Iaffner & DeLaHunty [57] ABSTRACT A pipe type electric cable in which the core comprising a central conductor surrounded by insulation which, in turn, is surrounded by a conductive screen, is received within a fluid-tight metal tube and is separated from and conductively connected to the inner surface of the pipe by one or more skid wires wound around the screen, each of which skid wires comprises one or more metal cords extending longitudinally of the skid wire and surrounded by a semi-conductivesynthetic resin which prevents contact of the cords with the inner surface of the pipe.

8 Claims, 3 Drawing Figures U.S. Patent Oct. 28, 1975 1 SKID WIRE FOR PIPE TYPE ELECTRIC CABLES The present invention relates to pipe type electric cables, especially for high voltages, which comprise a core including a central conductor surrounded by one or more layers of insulation and an outer electrostatic screen, a helically wound wire or wires, known in the art as skid wires, and surrounding the electrostatic screen a fluid-tight, rigid metal tube or pipe loosely receiving the core with the skid wires therearound. Such cables do not-have a sheath integral with, or tightly surrounding, the insulation or electrostatic screen of the core.

The skid wires serve, during the laying or installation operations, as well as during the thermal cycles, to mechanically protect the cable core. In addition, they assist in reducing the possibility of abrasion of the inner surface of the pipe during installation, and they conductively interconnect the electrostatic screen of the core with the pipe.

Up to the present time, four main types of skid wires were known, namely:

1. Bare metal skid wires of good conductivity and preferably made of copper, bronze, brass, zinc, stainless steel or aluminum.

2. Skid wires of dielectric or insulating material, such as a highly resistant synthetic resin, usually linear polyethylene or nylon. A

3. Skid wires of dielectric or insulating material with helically wound, exterior metal tapes.

4. Composite skid wires of insulating material with metal conductors embedded therein and extending from surface to surface thereof.

As is known, the metal skid wires set forth in (l) hereinbefore have a high friction coefficient. Therefore, they cause substantial abrasion of the inner surface of the pipe, and this causes problems, for example, in the use of aluminum pipes as the outer pipe. The employment of aluminum pipe would be of substantial interest because, by using such pipes, the losses due to eddy currents, with the same core, would be much lower than with iron tubes and, therefore, a greater current carrying capacity would be obtained.

In addition, such metal skid wires limit the maximum core length which can be drawn in a pipe, when it is taken into account that the whole drawing strain is transferred to the core conductor.

Dielectric skid wires of the type described in (2) hereinbefore find use only in very special cases, but not with metal pipes, because they do not provide the necessary electrical interconnection of the electrostatic screen of the core with the pipe. Such dielectric skid wires have, moreover, the disadvantage that they do not offer a sufficient mechanical resistance to the longitudinal stresses occurring during drawing and, therefore, they are subjected to rupturing and to consequent pulling out of the broken portions.

Dielectric skid wires of the type described in (3) hereinbefore obviously have a greater friction coefficient than polyethylene or nylon and, therefore, there is always the risk of undesirable abrasion of the inner surface of the pipe.

Although skid wires of the type described in (4) hereinbefore provide thenecessary electrical interconnection between the screen of the core and the pipe, the metal parts extending from the surface of the skid wire cause the undesirable effects of a high friction coefficient on the inner surface of the pipe. Moreover, said composite skid wires are difficult to manufacture, because of the difficulties encountered in finding a resin of low friction coefficient which is able to firmly adhere to the metal.

One object of the present invention is that of eliminating the disadvantages encountered with cables comprising conventional skid wires and at the same time, to provide cables comprising skid wires which may be easily manufactured.

According to the invention, the core of the cable is provided with skid wires which have a low friction coefficient, which provide very good electrical contact between the electrostatic screen of the core and the pipe, and which provide substantial resistance to the longitudinal drawing strains which occur during the installation of the core in the pipe.

More particularly, the object of the present invention is an electric cable of the pipe type, provided with one or more skid wires helically wound up on the electrostatic screen of the core, characterized in that each skid wire is constituted by an elongated element of synthetic resin made semi-conductive by means of an appropriate filler and one or more metallic cords surrounded by the resin and extending for the full length of said element.

Such object and other objects of the invention will be apparent from the following detailed description of a presently. preferred embodiment of the invention, which description should beconsidered in conjunction with the accompanying drawing in which:

FIG. 1 is a perspective view, partly in section, of a pipe cable constructed in accordance with the invention;

FIG. 2 shows, in enlarged scale, a perspective view of aportion of the skid wire of the cable shown in FIG. 1; and

FIG. 3 is a longitudinal section of the skid wire shown in FIGS. 1 and 2 and is taken along the line 3-3 indicated in FIG. 2.

FIG. 1 illustrates a screened cable core 10 laid in a pipe ll. The core 10 comprises a central stranded conductor l2, surrounded by a first conductive screen 13 which, in turn, is surrounded by layers of insulation 14.

The insulation 14 is surrounded by a second conductive, electrostatic screen 15. The screen 15 is of the conventional type, namely, is constituted by a metallic tape wound helically around the insulation 14.

Two skid wires 16 and 17 are wound helically around the electrostatic screen 15. Although two skid wires 16 and 17 have been shown, it will be apparent to those skilled in the art that the number of the skid wires applied can be different, for example, greater than two or only one skid wire. In case only one skid wire is used, said skid wire, for a given size of core 10, preferably will be applied at a pitch corresponding to one half of the pitch normally employed when two skid wires are used.

As illustrated in FIG. 2, each skid wire 16 and 17 is constituted by a tape or elongated element 18 of synthetic resin which is made semi-conductive by means of appropriate fillers. Preferably, the synthetic resin is high density polyethylene (friction coefficient on metal 0.25), polyvinyl chloride (friction coefiicient on metal 0.36) or nylon (friction coefiicient on metal 0.l9 0.20). The filler used to make the synthetic resin semiconductive preferably is carbon black.

Three

metal cords

19, 20 and 21,- for example, of stranded wires and preferably made of steel, extend longitudinallyof the element 18 and are completely embedded in, or surrounded by, the resin of the element 18 so that the

cords

19, 20 and 21 cannot engage the inner surface of the pipe 11. Although steel is preferred asthe metal for the

cords

19, 20 and 21, because of its strength, other metals may be used. The

cords

19, 20 and 21 run parallel to each other and are spaced apart from one another from one end of the skid wire to the other. The number of cords may be different from three and, for example, only one steel cord or more than three cords could be used.

Each skid wire 16 and 17 can have the shape indicated in FIG. 2, namely, can be D-shaped in crosssection. Thus, the skid wire 16 has a surface 22 of semiconductive resin which contacts the electrostatic screen 15 of the core 10. Said surface 22 is substantially flat but, if desired, can beconcave which permits it to adhere better to the electrostatic screen 15. The

lower surface 23 of the skid wire 16, also of semiconductive synthetic resin, is rounded so as to provide the conventional D-shape, but it could have another appropriate shape. For example, the surface 23 could be a flat surface, and, if desired, the skid ,wire 16 could rectangular in cross-section. Moreover, the skid wires 16 and 17 can be the same or different, provided that their maximum thickness s (FlGQ3) is the same.

The production of the skid wires 16 and 17 according to the invention is very easy, since it consists in the mere extrusion of the synthetic resin 18, chosen as desired and appropriately loaded, on the steel cord or

cords

19, 20 and 21 to be embedded.

Owing to the low friction coefficient of such skid wires 16 and 17, at the most preferably about 0.20 (dynamic friction coefficient on metals), with respect to the friction coefficient of "the skid wires of the prior art, which, as an average ranges from 0.5 to 0.6, the core can be easily drawn into the pipe 11 during installation, and can freely slide in the pipe 11 during thermal cycles, without causing significant abrasion of the inner surface 23 of the pipe 11.

In addition, the semi-conductive characteristic of the loaded synthetic resin of the elongated element 18 ensures a good conductive connection' between the screen and the pipe 11; The resistivity of the resin should be at least 1000 ohms/m. and, preferably, is in the range from l000-l0,000 ohms/m.

The cords l9, 20'and 21 embedded-in theresin element l8 cooperate with the latter by taking the longitudinal stresses exerted 'on the skid wires16 and 17 during drawing in operations for laying the core 10 in the P p Although a preferred embodiment of the present invention has been described and illustrated, it will be understood by those skilled 'in the art that various modifications may be made without departing from the principles of the invention,

What is claimed is:

1. An electric cable member for a pipe type cable comprising an electric conductor surrounded by insulation, a conductive screen around said insulation, and at least one skid wire wound around and in contact with said screen, said skid wire comprising at least one metal wire extending longitudinally of said skid wire for the length thereof and surrounded by a semi-conductive synthetic resin.

2. An electric cable member as set forth in claim 1, wherein said semi-conductive synthetic resin is a resin selected from the group consisting of high density polyethylene, polyvinyl chloride and nylon loaded with carbon black.

3. An electric cable member as set forth in claim 2, wherein said semi-conductive synthetic resin has a resistivity in the range from 1000 to 10,000 ohms per meter.

, 4. An electric cable member as set forth in claim 1, wherein said metal wire is stranded.

5. An electric cable member as set forth in claim 4, wherein the metal of said metal wire is steel.

6. An electric cable member as set forth in claim 1,

wherein said skid wire comprises at least three stranded extending longitudinally of said skid wire for the length thereof and surrounded by a semi-conductive synthetic resin.

Claims

4. An electric cable member as set forth in claim 1, wherein said metal wire is stranded.

6. An electric cable member as set forth in claim 1, wherein said skid wire comprises at least three stranded metal wires.

7. An electric cable member as set forth in claim 6, wherein said metal wires are parallel to and spaced from each other.

8. A pipe cable comprising a metal pipe, a cable core within said pipe, said core comprising an electric conductor surrounded by insulation and a conductive screen around said insulation, and at least one skid wire wound around said screen and intermediate it and said pipe, said skid wire comprising at least one metal wire extending longitudinally of said skid wire for the length thereof and surrounded by a semi-conductive synthetic resin.